High voltage battery composed of anode limited electrochemical cells

ABSTRACT

An electrochemical storage device including a plurality of electrochemical cells connected electrically in series. Each cell includes an anode electrode, a cathode electrode and an aqueous electrolyte. The charge storage capacity of the anode electrode is less than the charge storage capacity of the cathode.

FIELD

The present invention is directed to ensembles of electrochemical cellsand in particular to hybrid energy storage devices.

BACKGROUND

Small renewable energy harvesting and power generation technologies(such as solar arrays, wind turbines, micro sterling engines, and solidoxide fuel cells) are proliferating, and there is a commensurate strongneed for intermediate size secondary (rechargeable) energy storagecapability. Energy storage batteries for these stationary applicationstypically store between 1 and 50 kWh of energy (depending on theapplication) and have historically been based on the lead-acid (Pb acid)chemistry. The batteries typically comprise a number of individual cellsconnected in series and parallel to obtain the desired system capacityand bus voltage.

For vehicular and stationary storage applications, it is not unusual tohave batteries with bus voltages in the hundreds or thousands of volts,depending on application. In these cases, where many units are connectedelectrically in series, there is typically an inherent need for thesecells to be as similar to each other as possible. In the event that thecells are not similar enough, a cell-level monitoring and controllingcircuit is commonly necessary. If some set of cells in a string of cellshave lower charge capacity than others in the same string, the lowercapacity cells will reach an overcharge/undercharge condition duringfull discharge or charge of the string. These lower capacity cells willbe de-stabilized (typically due to electrolyte corrosion reactions),resulting in diminished lifetime performance of the battery. This effectis common in many battery chemistries and is seen prominently in theLi-ion battery and in the supercapacitor pack. In these systems, costlyand intricate cell-level management systems are needed if the cells arenot produced to exacting (and expensive) precision.

SUMMARY

An embodiment relates to an electrochemical storage device including aplurality of electrochemical cells connected electrically in series.Each cell includes an anode (negative) electrode, a cathode (positive)electrode and an aqueous electrolyte. The charge storage capacity of theanode electrode is less than the charge storage capacity of the cathode.

Another embodiment relates to a method of operating an electrochemicalenergy storage device. The method includes charging a plurality ofaqueous electrolyte electrochemical cells connected electrically inseries. The water in the aqueous electrolyte electrolyzes to formhydrogen and OH⁻ species at an anode electrode of at least one of theplurality of cells when a charge storage capacity of the anode electrodeof the at least one cell is exceeded on charging the at least one cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an electrochemical cell according to an embodiment.

FIG. 2 is a schematic illustration of an electrochemical cell accordingto an embodiment of the invention. The electrochemical cell may bestacked in a bipolar or prismatic stack configuration.

FIG. 3 is a schematic illustration of an electrochemical devicecomprising a bipolar stack of electrochemical cells according to anembodiment of the invention.

FIG. 4A illustrates an electrochemical energy storage system accordingto embodiment.

FIG. 4B illustrates an electrochemical energy storage system accordingto another embodiment.

FIG. 5 is a plot of battery capacity as a function of charge/dischargecycles.

DETAILED DESCRIPTION

It would be very useful to have batteries that can be built with cellsthat have a higher cell-to-cell charge storage capacity variationwithout sacrificing the integrity of the pack. The inventor hasdiscovered an aqueous electrolyte electrochemical cell that is able toself-regulate using internal electrochemical reactions upon overcharge.This self-regulation allows for high voltage strings of cells to bemanufactured with a high tolerance for cell-to-cell charge capacityvariation. Preferably, but not necessarily, the system lacks a celllevel voltage monitoring and control circuit. Thus, the cell levelvoltage is not monitored or controlled.

Without being bound by any particular theory, the inventor believes thatthe mechanism of self-regulation is the local electrolysis of theaqueous electrolyte that takes place at the anode electrode. Aselectrolysis occurs, a small amount of hydrogen is generated along withOH⁻ species. The OH⁻ species locally increase the pH, thereby pushingthe voltage stability window of electrolyte in the immediate vicinity ofthe anode to a lower value. This subsequently eliminates the continuedevolution of hydrogen. When the battery is allowed to discharge, it isthought that this small amount of hydrogen recombines with local OH⁻ tore-form water, or instead diffuses to the cathode side of the cell,where it can be similarly consumed. The inventor has discovered that theuse of an anode electrode of a material with a high overpotential forhydrogen evolution from water, such as carbon, combined with the localelectrolysis and recombination of the aqueous electrolyte allows for anelectrode environment that is highly tolerant to overcharge.

An embodiment of the invention includes an electrochemical storagedevice that includes cells electrically connected in series having awider as-manufactured cell-to-cell variation in charge storage capacitythan conventional charge storage devices. In this embodiment, cells witha lower charge storage capacity in the same string of cells charge tohigher potentials during cycling. When this happens, the effectdescribed above is believed to occur with no long-term detriment to thecell string.

In an embodiment, the electrochemical storage device is a hybridelectrochemical energy storage system in which the individualelectrochemical cells include a pseudocapacitive or double-layercapacitor electrode (e.g., anode) coupled with an active electrode. Inthese systems, the capacitor electrode stores charge through areversible nonfaradiac reaction of alkali (e.g., Li, Na, K, etc.) or Cacations on the surface of the electrode (double-layer) and/orpseudocapacitance, while the active electrode undergoes a reversiblefaradic reaction in a transition metal oxide or a similar material thatintercalates and deintercalates alkali or Ca cations similar to that ofa battery.

An example of a Li-based system has been described by Wang, et al.,which utilizes a spinel structure LiMn₂O₄ battery electrode, anactivated carbon capacitor electrode, and an aqueous Li₂SO₄ electrolyte.Wang, et al., Electrochemistry Communications, 7:1138-42(2005). In thissystem, the negative anode electrode stores charge through a reversiblenonfaradiac reaction of Li-ion on the surface of an activated carbonelectrode. The positive cathode electrode utilizes a reversible faradiacreaction of Li-ion intercalation/deintercalation in spinel LiMn₂O₄. Adifferent system is disclosed in U.S. patent application Ser. No.12/385,277, filed Apr. 3, 2009, hereby incorporated by reference in itsentirety. In this system, the cathode electrode comprises a materialhaving a formula A_(x)M_(y)O_(z). A is one or more of Li, Na, K, Be, Mg,and Ca, x is within a range of 0 to 1 before use and within a range of 0to 10 during use. M comprises any one or more transition metals, y iswithin a range of 1 to 3 and z is within a range of 2 to 7. The anodeelectrode comprises activated carbon and the electrolyte comprises SO₄²⁻, NO₃ ⁻, ClO₄ ⁻, PO₄ ³⁻, CO₃ ²⁻, Cl⁻, or OH⁻ anions. Preferably, thecathode electrode comprises a doped or undoped cubic spinel λ-MnO₂-typematerial or a NaMn₉O₁₈ tunnel structured orthorhombic material, theanode electrode comprises activated carbon and the electrolyte comprisesNa₂SO₄ solvated in water.

FIG. 1 is a schematic illustration of an exemplary electrochemical cell102 according to an embodiment. The cell 102 includes a cathode sidecurrent collector 1 in contact with a cathode electrode 3. The cathodeelectrode 3 is in contact with an aqueous electrolyte solution 5, whichis also in contact with an anode electrode 9. The cell 102 also includesa separator 7 located in the electrolyte solution 5 at a point betweenthe cathode electrode 3 and the anode electrode 9. The anode electrodeis also in contact with an anode side current collector 11. In FIG. 1,the components of the exemplary cell 102 are shown as not being incontact with each other. The cell 102 was illustrated this way toclearly indicate the presence of the electrolyte solution 5 relative toboth electrodes. However, in actual embodiments, the cathode electrode 3is in contact with the separator 7, which is in contact with the anodeelectrode 9.

In this embodiment, the cell 102 is “anode limited”. That is, the chargestorage capacity of the anode electrode 9 is less than that of thecathode electrode 3. The charge storage capacity of an electrode is theproduct of the mass of the electrode and the specific capacity (in unitsof Ah/kg) of the electrode material. Thus, in an anode limited cell, themass of the active cathode material multiplied by the usable specificcapacity of the cathode material is greater than the mass of the activeanode material multiplied by the useable specific capacity of the anodematerial. Preferably, the storage capacity of the anode electrode 9 is50-90%, such as 75-90% of the charge storage capacity of the cathodeelectrode 3. Preferably, the anode electrode 9 is made from a materialthat is corrosion resistant (resistant to the hydrogen formed byelectrolysis) at the charging voltage as will be discussed below. Amethod according to an embodiment includes charging the energy storagesystem 100 at a voltage 1.5 times greater and/or 0.8 volts higher than avoltage at which electrolysis of the water at the anode electrode of thecells is initiated, without inducing corrosion of the anode electrodematerial.

FIG. 2 illustrates another embodiment of an electrochemical cell 102.The electrochemical cell 102 includes an anode electrode 104, a cathodeelectrode 106 and a separator 108 between the anode electrode 104 andthe cathode electrode 106. The electrochemical cell 102 also includes anelectrolyte located between the anode electrode 104 and the cathodeelectrode 106. In an embodiment, the separator 108 may be porous withelectrolyte located in the pores. The electrochemical cell 102 may alsoinclude a graphite sheet 110 that acts as a current collector for theelectrochemical cell 102. Preferably, the graphite sheet 110 isdensified. In an embodiment, the density of the graphite sheet 110 isgreater than 0.6 g/cm³. The graphite sheet 110 may be made from, forexample, exfoliated graphite. In an embodiment, the graphite sheet 110may include one or more foil layers. Suitable materials for the anodeelectrode 104, the cathode electrode 106, the separator 108 and theelectrolyte are discussed in more detail below.

The anode electrode 104, the cathode electrode 106, the separator 108and the graphite sheet current collector 110 may be mounted in a frame112 which seals each individual cell. The frame 112 is preferably madeof an electrically insulating material, for example, an electricallyinsulating plastic or epoxy. The frame 112 may be made from preformedrings, poured epoxy or a combination of the two. In an embodiment, theframe 112 may comprise separate anode and cathode frames. In anembodiment, the graphite sheet current collector 110 may be configuredto act as a seal 114 with the frame 112. That is, the graphite sheetcurrent collector 110 may extend into a recess in the frame 112 to actas the seal 114. In this embodiment, the seal 114 prevents electrolytefrom flowing from one electrochemical cell 102 to an adjacentelectrochemical cell 102. In alternative embodiments, a separate seal114, such as a washer or gasket, may be provided such that the graphitesheet current collector does not perform as a seal.

In an embodiment, the electrochemical cell is a hybrid electrochemicalcell. That is, the cathode electrode 106 in operation reversiblyintercalates alkali metal cations and the anode electrode 104 comprisesa capacitive electrode which stores charge through either (1) areversible nonfaradiac reaction of alkali metal cations on a surface ofthe anode electrode or (2) a pseudocapacitive electrode which undergoesa partial charge transfer surface interaction with alkali metal cationson a surface of the anode electrode.

Individual device components may be made of a variety of materials asfollows.

Anode

The anode may, in general, comprise any material capable of reversiblystoring Na-ions (and/or other alkali or alkali earth ions) throughsurface adsorption/desorption (via an electrochemical double layerreaction and/or a pseudocapacitive reaction (i.e. partial chargetransfer surface interaction)) and be corrosion/hydrogen resistant inthe desired voltage range. In an embodiment, the anodes are made ofactivated carbon (which is corrosion free; that is, not damaged byevolved hydrogen). Preferably, the anode electrode comprises highsurface area (e.g., activated) carbon that has been modified to havemore than 120 F/g (e.g., 120 to 180 F/g) in 1 M Na₂SO₄ under anodicbiasing conditions. Preferably, the activated carbon anode ispseudocapacitive and is configured to operate in a voltage range of −1to 0.8 volts SHE. Alternative anode materials include graphite,mesoporous carbon, carbon nanotubes, disordered carbon, Ti-oxide (suchas titania) materials, V-oxide materials, phospho-olivine materials,other suitable mesoporous ceramic materials, and combinations thereof.

Optionally, the anode electrode may be in the form of a composite anodecomprising activated carbon, a high surface area conductive diluent(such as conducting grade graphite, carbon blacks, such as acetyleneblack, non-reactive metals, and/or conductive polymers), a binder, suchas PTFE, a PVC-based composite (including a PVC-SiO₂ composite),cellulose-based materials, PVDF, other non-reactive non-corrodingpolymer materials, or a combination thereof, plasticizer, and/or afiller. The composite anode electrode, as with a single material anodeelectrode, should be corrosion/hydrogen resistant in the desired voltagerange. In an embodiment, the anode electrode comprises an alkalititanate compound that reversibly interacts with alkali or alkali earthions via a pseudocapacitive or intercalative reaction mechanism, such assodium or lithium titanate. The alkali titanate may be, for example, inthe form of nanocrystals on the surface of the anode or intercalatedinto the anode.

Cathode

Any suitable material comprising a transition metal oxide, sulfide,phosphate, or fluoride can be used as active cathode materials capableof reversible alkali and/or alkali earth ion, such as Na-ionintercalation/deintercalation. Materials suitable for use as activecathode materials in embodiments of the present invention preferablycontain alkali atoms, such as sodium, lithium, or both, prior to use asactive cathode materials. It is not necessary for an active cathodematerial to contain Na and/or Li in the as-formed state (that is, priorto use in an energy storage device). However, for devices in which use aNa-based electrolyte, Na cations from the electrolyte should be able toincorporate into the active cathode material by intercalation duringoperation of the energy storage device. Thus, materials that may be usedas cathodes in embodiments of the present invention comprise materialsthat do not necessarily contain Na or other alkali in an as-formedstate, but are capable of reversible intercalation/deintercalation of Naor other alkali-ions during discharging/charging cycles of the energystorage device without a large overpotential loss.

In embodiments where the active cathode material contains alkali-atoms(preferably Na or Li) prior to use, some or all of these atoms aredeintercalated during the first cell charging cycle. Alkali cations froma sodium based electrolyte (overwhelmingly Na cations) arere-intercalated during cell discharge. This is different than nearly allof the hybrid capacitor systems that call out an intercalation electrodeopposite activated carbon. In most systems, cations from the electrolyteare adsorbed on the anode during a charging cycle. At the same time, thecounter-anions, such as hydrogen ions, in the electrolyte intercalateinto the active cathode material, thus preserving charge balance, butdepleting ionic concentration, in the electrolyte solution. Duringdischarge, cations are released from the anode and anions are releasedfrom the cathode, thus preserving charge balance, but increasing ionicconcentration, in the electrolyte solution. This is a differentoperational mode from devices in embodiments of the present invention,where hydrogen ions or other anions are preferably not intercalated intothe cathode active material and/or are not present in the device. Theexamples below illustrate cathode compositions suitable for Naintercalation. However, cathodes suitable for Li, K or alkali earthintercalation may also be used.

Suitable active cathode materials may have the following general formuladuring use: A_(x)M_(y)O_(z), where A is Na or a mixture of Na and one ormore of Li, K, Be, Mg, and Ca, where x is within the range of 0 to 1,inclusive, before use and within the range of 0 to 10, inclusive, duringuse; M comprises any one or more transition metal, where y is within therange of 1 to 3, inclusive; preferably within the range of 1.5 and 2.5,inclusive; and O is oxygen, where z is within the range of 2 to 7,inclusive; preferably within the range of 3.5 to 4.5, inclusive.

In some active cathode materials with the general formulaA_(x)M_(y)O_(z), Na-ions reversibly intercalate/deintercalate during thedischarge/charge cycle of the energy storage device. Thus, the quantityx in the active cathode material formula changes while the device is inuse.

In some active cathode materials with the general formulaA_(x)M_(y)O_(z), A comprises at least 50 at % of at least one or more ofNa, K, Be, Mg, or Ca, optionally in combination with Li; M comprises anyone or more transition metal; O is oxygen; x ranges from 3.5 to 4.5before use and from 1 to 10 during use; y ranges from 8.5 to 9.5 and zranges from 17.5 to 18.5. In these embodiments, A preferably comprisesat least 51 at % Na, such as at least 75 at % Na, and 0 to 49 at %, suchas 0 to 25 at %, Li, K, Be, Mg, or Ca; M comprises one or more of Mn,Ti, Fe, Co, Ni, Cu, V, or Sc; x is about 4 before use and ranges from 0to 10 during use; y is about 9; and z is about 18.

In some active cathode materials with the general formulaA_(x)M_(y)O_(z), A comprises Na or a mix of at least 80 atomic percentNa and one or more of Li, K, Be, Mg, and Ca. In these embodiments, x ispreferably about 1 before use and ranges from 0 to about 1.5 during use.In some preferred active cathode materials, M comprises one or more ofMn, Ti, Fe, Co, Ni, Cu, and V, and may be doped (less than 20 at %, suchas 0.1 to 10 at %; for example, 3 to 6 at %) with one or more of Al, Mg,Ga, In, Cu, Zn, and Ni.

General classes of suitable active cathode materials include (but arenot limited to) the layered/orthorhombic NaMO₂ (birnessite), the cubicspinel based manganate (e.g., MO₂, such as λ-MnO₂ based material where Mis Mn, e.g., Li_(x)M₂O₄ (where 1≦x<1.1) before use and Na₂Mn₂O₄ in use),the Na₂M₃O₇ system, the NaMPO₄ system, the NaM₂(PO₄)₃ system, theNa₂MPO₄F system, and the tunnel-structured orthorhombic NaM₉O₁₈, where Min all formulas comprises at least one transition metal. Typicaltransition metals may be Mn or Fe (for cost and environmental reasons),although Co, Ni, Cr, V, Ti, Cu, Zr, Nb, W, Mo (among others), orcombinations thereof, may be used to wholly or partially replace Mn, Fe,or a combination thereof. In embodiments of the present invention, Mn isa preferred transition metal. In some embodiments, cathode electrodesmay comprise multiple active cathode materials, either in a homogenousor near homogenous mixture or layered within the cathode electrode.

In some embodiments, the initial active cathode material comprisesNaMnO₂ (birnassite structure) optionally doped with one or more metals,such as Li or Al.

In some embodiments, the initial active cathode material comprisesλ-MnO₂ (i.e., the cubic isomorph of manganese oxide) based material,optionally doped with one or more metals, such as Li or Al.

In these embodiments, cubic spinel λ-MnO₂ may be formed by first forminga lithium containing manganese oxide, such as lithium manganate (e.g.,cubic spinel LiMn₂O₄) or non-stoichiometric variants thereof. Inembodiments which utilize a cubic spinel λ-MnO₂ active cathode material,most or all of the Li may be extracted electrochemically or chemicallyfrom the cubic spinel LiMn₂O₄ to form cubic spinel λ-MnO₂ type material(i.e., material which has a 1:2 Mn to O ratio, and/or in which the Mnmay be substituted by another metal, and/or which also contains analkali metal, and/or in which the Mn to O ratio is not exactly 1:2).This extraction may take place as part of the initial device chargingcycle. In such instances, Li-ions are deintercalated from the as-formedcubic spinel LiMn₂O₄ during the first charging cycle. Upon discharge,Na-ions from the electrolyte intercalate into the cubic spinel λ-MnO₂.As such, the formula for the active cathode material during operation isNa_(y)Li_(x)Mn₂O₄ (optionally doped with one or more additional metal asdescribed above, preferably Al), with 0<x<1, 0<y<1, and x+y≦1.1.Preferably, the quantity x+y changes through the charge/discharge cyclefrom about 0 (fully charged) to about 1 (fully discharged). However,values above 1 during full discharge may be used. Furthermore, any othersuitable formation method may be used. Non-stoichiometric Li_(x)Mn₂O₄materials with more than 1 Li for every 2 Mn and 40 atoms may be used asinitial materials from which cubic spinel λ-MnO₂ may be formed (where1≦x<1.1 for example). Thus, the cubic spinel λ-manganate may have aformula Al_(z)Li_(x)Mn_(2-z)O₄ where 1≦x<1.1 and 0≦z<0.1 before use, andAl_(z)Li_(x)Na_(y)Mn₂O₄ where 0≦x<1.1, 0≦x<1, 0≦x+y<1.1, and 0≦z<0.1 inuse (and where Al may be substituted by another dopant).

In some embodiments, the initial cathode material comprises Na₂Mn₃O₇,optionally doped with one or more metals, such as Li or Al.

In some embodiments, the initial cathode material comprises Na₂FePO₄F,optionally doped with one or more metals, such as Li or Al.

In some embodiments, the cathode material comprises orthorhombicNaM₉O₁₈, optionally doped with one or more metals, such as Li or Al.This active cathode material may be made by thoroughly mixing Na₂CO₃ andMn₂O₃ to proper molar ratios and firing, for example at about 800° C.The degree of Na content incorporated into this material during firingdetermines the oxidation state of the Mn and how it bonds with O₂locally. This material has been demonstrated to cycle between0.33<x<0.66 for Na_(x)MnO₂ in a non-aqueous electrolyte. Alternatively,the cathode material comprises LiMn₂O₄ and the electrolyte comprisesLi₂SO₄.

Optionally, the cathode electrode may be in the form of a compositecathode comprising one or more active cathode materials, a high surfacearea conductive diluent (such as conducting grade graphite, carbonblacks, such as acetylene black, non-reactive metals, and/or conductivepolymers), a binder, a plasticizer, and/or a filler. Exemplary bindersmay comprise polytetrafluoroethylene (PTFE), a polyvinylchloride(PVC)-based composite (including a PVC-SiO₂ composite), cellulose-basedmaterials, polyvinylidene fluoride (PVDF), hydrated birnassite (when theactive cathode material comprises another material), other non-reactivenon-corroding polymer materials, or a combination thereof. A compositecathode may be formed by mixing a portion of one or more preferredactive cathode materials with a conductive diluent, and/or a polymericbinder, and pressing the mixture into a pellet. In some embodiments, acomposite cathode electrode may be formed from a mixture of about 50 to90 wt % active cathode material, with the remainder of the mixturecomprising a combination of one or more of diluent, binder, plasticizer,and/or filler. For example, in some embodiments, a composite cathodeelectrode may be formed from about 80 wt % active cathode material,about 10 to 15 wt % diluent, such as carbon black, and about 5 to 10 wt% binder, such as PTFE.

One or more additional functional materials may optionally be added to acomposite cathode to increase capacity and replace the polymeric binder.These optional materials include but are not limited to Zn, Pb, hydratedNaMnO₂ (birnassite), and Na₄Mn₉O₁₈ (orthorhombic tunnel structure). Ininstances where hydrated NaMnO₂ (birnassite) and/or hydratedNa_(0.44)MnO₂ (orthorhombic tunnel structure) is added to a compositecathode, the resulting device has a dual functional material compositecathode. A cathode electrode will generally have a thickness in therange of about 40 to 800 μm.

Current Collectors

In embodiments of the present invention, the cathode and anode materialsmay be mounted on current collectors. For optimal performance, currentcollectors are desirable that are electronically conductive andcorrosion resistant in the electrolyte (aqueous Na-cation containingsolutions, described below) at operational potentials.

For example, an anode current collector should be stable in a range ofapproximately −1.2 to −0.5 V vs. a standard Hg/Hg₂SO₄ referenceelectrode, since this is the nominal potential range that the anode halfof the electrochemical cell is exposed during use. A cathode currentcollector should be stable in a range of approximately 0.1 to 0.7 V vs.a standard Hg/Hg₂SO₄ reference electrode.

Suitable uncoated current collector materials for the anode side includestainless steel, Ni, NiCr alloys, Al, Ti, Cu, Pb and Pb alloys,refractory metals, and noble metals.

Suitable uncoated current collector materials for the cathode sideinclude stainless steel, Ni, NiCr alloys, Ti, Pb-oxides (PbO_(x)), andnoble metals.

Current collectors may comprise solid foils or mesh materials.

Another approach is to coat a metal foil current collector of a suitablemetal, such as Al, with a thin passivation layer that will not corrodeand will protect the foil onto which it is deposited. Such corrosionresistant layers may be, but are not limited to, TiN, CrN, C, CN, NiZr,NiCr, Mo, Ti, Ta, Pt, Pd, Zr, W, FeN, CoN, etc. These coated currentcollectors may be used for the anode and/or cathode sides of a cell. Inone embodiment, the cathode current collector comprises Al foil coatedwith TiN, FeN, C, or CN. The coating may be accomplished by any methodknown in the art, such as but not limited to physical vapor depositionsuch as sputtering, chemical vapor deposition, electrodeposition, spraydeposition, or lamination.

Electrolyte

Embodiments of the present invention provide a secondary (rechargeable)energy storage system which uses a water-based (aqueous) electrolyte,such as an alkali based (e.g., Li and/or Na-based) or alkaline earthbased aqueous electrolyte. Use of Na allows for use of much thickerelectrodes, much less expensive separator and current collectormaterials, and benign and more environmentally friendly materials forelectrodes and electrolyte salts. Additionally, energy storage systemsof embodiments of the present invention can be assembled in an open-airenvironment, resulting in a significantly lower cost of production.

Electrolytes useful in embodiments of the present invention comprise asalt dissolved fully in water. For example, the electrolyte may comprisea 0.1 M to 10 M solution of at least one anion selected from the groupconsisting of SO₄ ²⁻, NO₃ ⁻, ClO₄ ⁻, PO₄ ³⁻, CO₃ ²⁻, Cl⁻, and/or OH⁻.Thus, Na cation containing salts may include (but are not limited to)Na₂SO₄, NaNO₃, NaClO₄, Na₃PO₄, Na₂CO₃, NaCl, and NaOH, or a combinationthereof.

In some embodiments, the electrolyte solution may be substantially freeof Na. In these instances, cations in salts of the above listed anionsmay be an alkali other than Na (such as Li or K) or alkaline earth (suchas Ca, or Mg) cation. Thus, alkali other than Na cation containing saltsmay include (but are not limited to) Li₂SO₄, LiNO₃, LiClO₄, Li₃PO₄,Li₂CO₃, LiCl, and LiOH, K₂SO₄, KNO₃, KClO₄, K₃PO₄, K₂CO₃, KCl, and KOH.Exemplary alkaline earth cation containing salts may include CaSO₄,Ca(NO₃)₂, Ca(ClO₄)₂, CaCO₃, and Ca(OH)₂, MgSO₄, Mg(NO₃)₂, Mg(ClO₄)₂,MgCO₃, and Mg(OH)₂. Electrolyte solutions substantially free of Na maybe made from any combination of such salts. In other embodiments, theelectrolyte solution may comprise a solution of a Na cation containingsalt and one or more non-Na cation containing salt.

Molar concentrations preferably range from about 0.05 M to 3 M, such asabout 0.1 to 1 M, at 100° C. for Na₂SO₄ in water depending on thedesired performance characteristics of the energy storage device, andthe degradation/performance limiting mechanisms associated with highersalt concentrations. Similar ranges are preferred for other salts.

A blend of different salts (such as a blend of a sodium containing saltwith one or more of an alkali, alkaline earth, lanthanide, aluminum andzinc salt) may result in an optimized system. Such a blend may providean electrolyte with sodium cations and one or more cations selected fromthe group consisting of alkali (such as Li or K), alkaline earth (suchas Mg and Ca), lanthanide, aluminum, and zinc cations.

The pH of the electrolyte may be neutral (e.g., close to 7 at roomtemperature, such as 6.5 to 7.5). Optionally, the pH of the electrolytemay be altered by adding some additional OH− ionic species to make theelectrolyte solution more basic, for example by adding NaOH other OH⁻containing salts, or by adding some other OH⁻ concentration-affectingcompound (such as H₂SO₄ to make the electrolyte solution more acidic).The pH of the electrolyte affects the range of voltage stability window(relative to a reference electrode) of the cell and also can have aneffect on the stability and degradation of the active cathode materialand may inhibit proton (H⁺) intercalation, which may play a role inactive cathode material capacity loss and cell degradation. In somecases, the pH can be increased to 11 to 13, thereby allowing differentactive cathode materials to be stable (than were stable at neutral pH7). In some embodiments, the pH may be within the range of about 3 to13, such as between about 3 and 6 or between about 8 and 13.

Optionally, the electrolyte solution contains an additive for mitigatingdegradation of the active cathode material, such as birnassite material.An exemplary additive may be, but is not limited to, Na₂HPO₄, inquantities sufficient to establish a concentration ranging from 0.1 mMto 100 mM.

Separator

A separator for use in embodiments of the present invention may comprisea woven or non-woven cotton sheet, PVC (polyvinyl chloride), PE(polyethylene), glass fiber or any other suitable material.

FIG. 3 illustrates embodiment of an electrochemical energy storagesystem 100. In this embodiment, the electrochemical energy storagesystem 100 comprises a bipolar stack 101 of electrochemical cells 102according to another embodiment. In contrast to conventional stacks ofelectrochemical cells which include separate anode side and cathode sidecurrent collectors, in one embodiment, the bipolar stack 100B operateswith a single graphite sheet current collector 110 located between thecathode electrode 106 of one electrochemical cell 102 and the anodeelectrode 104 of an adjacent electrochemical cell 102. Thus, bipolarstack 100B only uses half as many current collectors as the conventionalstack of electrochemical cells.

In an embodiment, the bipolar stack 101 is enclosed in an outer housing116 and provided with conducting headers 118 on the top and bottom ofthe bipolar stack 101. The headers 118 preferably comprise a corrosionresistant current collector metal, including but not limited to,aluminum, nickel, titanium and stainless steel. Preferably, pressure isapplied to the bipolar stack 101 when assembled. The pressure aids inproviding good seals to prevent leakage of electrolyte.

FIG. 4A illustrates an embodiment of an electrochemical energy storagesystem 100 according to an embodiment. The electrochemical energystorage system 100 includes a stack 101 of cells 102. The stack 101 ofcells 102 may include 2, 4, 6, 8, or more cells 102. The stack 101 maythen be enclosed in a housing 116. The top and bottom contacts 120extend out of the housing 116 and provide a path for electricity to flowin and out of the cell 102.

In this embodiment, the electrochemical energy storage system 100preferably includes multiple stacks 101 of cells 102. As illustrated,the electrochemical energy storage system 100 includes 8 stacks of cells102, however, any number of stacks 101, such as 1, 2, 3, 4, 5, 6, 7, 8or 10 may be fabricated. Larger electrochemical energy storage systems100 having 20, 40, 50, 100 or 1000 stacks may also be fabricated. In anembodiment, all of the cells 102 in a stack 101 are connected inparallel while the stacks 101 are connected to each other in series. Inother embodiments, one or more stacks 101 may be connected in parallel.In this manner, high voltages, such as hundreds or thousands of voltscan be generated.

FIG. 4B illustrates another embodiment of an electrochemical energystorage system 100. In this embodiment, two or more of theelectrochemical energy storage systems 100 illustrated in FIG. 4A areconnected in series. In this configuration, very large voltages may beconveniently generated. In an alternative embodiment, two or more of theelectrochemical energy storage systems 100 illustrated in FIG. 4A areconnected in parallel. In this configuration, large currents may beprovided at a desired voltage.

FIG. 5 shows data from a stack 101 of 10 cells 102 made withnon-perfectly matched units cycled for many cycles. The cathodeelectrode 3 was made from λ-MnO₂ and the anode electrode 9 was made fromactivated carbon. These cells are designed for 0.6 to 1.8V/celloperation. The anode electrode 9 had a charge storage capacity that was90% of the capacity of the cathode electrode 3. For the first 34 cycles,the stack 101 was charged at 18 volts (1.8 volts/cell). The stack 101was then charged at 19 volts (1.9 V/cell) for 11 cycles followed by 20volts (2.0V/cell) for 5 cycles. After 50 cycles, the data show that eventhough the aqueous cell voltage is higher than the expected stabilitywindow of water (1.23 V at 25 C), the stack 101 of cells 102 can bestably cycled. The data show no loss of function (no loss of capacity)through 50 cycles. This cannot be done for cells that are cathodelimited (where the overpotential condition manifests at the cathode 3).This is because if the cell 102 was cathode limited, there would beoxygen evolving at the cathode 3 that would contribute to significantactive material (metal oxide cathode) corrosion, leading to eventualfailure of the cell 102.

Although the foregoing refers to particular preferred embodiments, itwill be understood that the invention is not so limited. It will occurto those of ordinary skill in the art that various modifications may bemade to the disclosed embodiments and that such modifications areintended to be within the scope of the invention. All of thepublications, patent applications and patents cited herein areincorporated herein by reference in their entirety.

What is claimed is:
 1. A method of operating an electrochemical energystorage device, comprising charging a plurality of aqueous electrolyteelectrochemical cells connected electrically in series, such that acharge storage capacity of an anode electrode in at least one of theplurality of cells is exceeded on charging, wherein water in the aqueouselectrolyte electrolyzes to form hydrogen and OH⁻ species at the anodeelectrode of the at least one of the plurality of cells when the chargestorage capacity of the anode electrode of the at least one of theplurality of cells is exceeded on charging the at least one of theplurality of cells, wherein: each of the plurality of cells furthercomprises a cathode electrode; the charge storage capacity of the anodeelectrode is less than a charge storage capacity of the cathodeelectrode in each of the plurality of cells; each of the plurality ofcells is a secondary hybrid aqueous energy storage cell; the cathodeelectrode in operation reversibly intercalates alkali metal cations; andthe anode electrode is a capacitive electrode which stores chargethrough a nonfaradiac reaction of the alkali metal cations on a surfaceof the anode electrode or a pseudocapacitive electrode which undergoes apartial charge transfer surface interaction with the alkali metalcations on the surface of the anode electrode.
 2. The method of claim 1,wherein the charge storage capacity of the anode electrode is 50-90% ofthe charge storage capacity of the cathode electrode.
 3. The method ofclaim 1, wherein: the cathode electrode comprises a doped or undopedcubic spinel λ-MnO₂-type material or a NaMn₉O₁₈ tunnel structuredorthorhombic material, the anode electrode comprises activated carbon,the aqueous electrolyte comprises a combination of one or more alkalications and SO₄ anion species solvated in water, and a cell levelvoltage is not monitored or controlled.
 4. The method of claim 1,wherein: the OH⁻ species increase a pH proximal to the surface of theanode electrode; the increase in pH lowers a voltage stability window ofthe aqueous electrolyte, thereby reducing or eliminating furtherhydrogen evolution; and the hydrogen species formed on charging the atleast one of the plurality of cells combines with the OH⁻ species ondischarging of the at least one of the plurality of cells.
 5. The methodof claim 3, further comprising charging the electrochemical energystorage device at a voltage 1.5 times greater or 0.8 volts higher than avoltage at which electrolysis of the water at the anode electrode isinitiated.
 6. The method of claim 1, wherein the electrochemical energystorage device shows no loss of capacity after 50 charge-dischargecycles during which the charge storage capacity of the anode electrodein the at least one of the plurality of cells is exceeded on charging.7. The method of claim 4, wherein the anode electrode consistsessentially of activated carbon.